A fluidized bed process is a process such as mixing, agglomerating, granulating, pelletization, coating, drying or cooling wherein a bed of solid particles disposed in a vessel with a stream of air or gas passing upward through the particles at a rate great enough to set the solid particles in motion and affect the desired process. Typically, pressurized air or gas enters the fluidized bed vessel through numerous holes in a distributor plate at the bottom of the fluidized bed. The air or gas stream flows upward through the bed, causing the solid particles to be suspended in the stream while the desired process is on-going. Batch fluidized bed processing allows several process steps (e.g. mixing, drying, cooling, etc.) to be carried out in a single vessel or unit. The batch fluidized bed process assures uniformity of all product within a batch.
In a batch fluidized bed drying process, a heated stream of air flows up through the wet product disposed in a vessel and removes solvent from the wet product leaving a dryer, less dense particle without affecting the elemental nature of the product. While an air-blown Fluidized Bed Dryer (FBD) is typically an easy to operate and low cost option for drying many chemical or pharmaceutical products, there are notable disadvantages to using an air-blown FBD. For example, air blown FBDs are generally unsuitable for drying products that are sensitive to the moisture, impurities or oxygen contained within the air stream and unsuitable for drying products containing organic or flammable solvents. Specifically, there are many chemical and pharmaceutical products that tend to degrade when exposed to the oxygen or moisture contained in the drying air. In addition, there are many safety concerns associated with the introduction of oxygen during the drying process of various chemical or pharmaceutical products as the organic solvents from the wet product may be volatile in nature and may actually form combustible mixtures when exposed to the oxygen in the drying air. Other disadvantages associated with an air-blown FBD are the difficulty and expense of recovering the solvent from the air stream that was extracted during the drying process.
An alternative to an air-blown FBD is to use an inert gas or moisture free fluidizing atmosphere in lieu of standard air for drying the chemical or pharmaceutical products. For example, fluidized bed drying processes that use an organic solvent typically require an inert gas, such as nitrogen, to replace the air as the medium of fluidization. However, using the inert gas atmospheres are significantly more expensive than using standard air and thus, recycling of the inert fluidizing gas is an adopted practice. Recycling of the inert fluidizing gas typically requires removal of the volatile solvents and other impurities picked up by the gas stream during the drying of the product. Existing FBD systems have used nitrogen gas as the inert fluidizing gas and a solvent removal process downstream of the drying vessel or tank to continuously remove the solvent from the entire stream exiting the FBD with the solvent-free or purified nitrogen gas being recirculated or recycled back to the FBD. Unfortunately, due to the high flow rate of the inert gas both into and from the FBD, the cost and size of the solvent recovery system presents significant economic disadvantages to using inert gas blown FBD to dry many chemical and pharmaceutical products.
Another alternative to using an air-blown FBD or inert gas blown FBD is to a vacuum drier. In about 1989, the first vacuum fluidized bed system was presented wherein a fluidized bed was generated and sustained under vacuum conditions, thereby eliminating the need for the inert gas. In fact, vacuum driers are currently used today for many product drying applications where the products are sensitive to the moisture, impurities or oxygen or applications where the solvents are highly volatile and flammable. Unfortunately, vacuum driers are very expensive both from a capital expense standpoint as well as an operating cost standpoint. Also, most drying process that use vacuum driers have much longer drying times than an air-blown FBD or inert gas blown FBD. Also, solvent recovery in a vacuum drier based drying process is a more difficult and more expensive proposition compared to solvent recovery in either air-blown or nitrogen blown FBD systems. Although vacuum systems can lower the boiling point of the solvent to be removed, a vacuum environment can insulate the thermal heat transfer and inhibit mass transfer in bulk without a carrier gas. Due to the difficulties to keep seals from leaking thru rotating equipment, vacuum driers seldom has ability to agitate the powders, resulting in poor heat and mass transfer efficiencies.
What is needed therefore is an economically attractive and cost-effective solution for batch fluidized bed processing of products in an inert environment that allows for the efficient recovery of solvents. Such solution demands reductions in both operating costs of the batch fluidized bed process and the associated capital costs (i.e. size of equipment) compared to existing solutions without adverse impact to process cycles or times.